How can I add more than one animation to the same model?
Just adding this as an alternative approach for future. Blender has this thing called Actions that are essentially separated animation timelines. You can have any number of actions in your blend file for every animation you want to create (idle, idle2, walk, walk2...) and you can add, copy and delete them whichever way you like without them interfering with each other in any way.

You can then mix actions into longer animations (walk+hand wave) using the NLA editor, line them up as one long, continuous animation like you're doing now, or if you're targeting Unity or somesuch, FBX export can save all the actions along with the file. No need to manually set the frame ranges later.

You can get it done the way you're doing, sure. And actions are a bit confusing to work with first. But I think it's the easier route in the long run.